You said, But,if the first was 640X480, i can't over scale and there is always black borders.

If you start a "NEW Project" using Flip Phone settings you can insert 640 x 480 footage to the timeline without any borders.

You can choose either 29.97 frames per second or 30 depending on which flip phone settings you select.

If your footage is 60 fps you could also select the "DV" format instead.

You could produce the 640 x 480 footage as it's own project. That way you could produce it as a 640 x 480 project without any black borders.

However, when you play it back on a wide screen it won't fill the entire screen .On a computer it would fill the player but the player and the video still would be smaller that the screen.

Playing it at full screen would force the player to add the black borders because of the aspect ratio.

Bottom line, you cannot fit a Squar"ish" shaped peg in a Rectangular hole without borders. A 640 x 480 video would need to be stretched out of proportion or over-scaled and cropped to fill a Wide screen16 x 9 Aspect ratio screen.